The present invention relates to safety pressure relief devices and more particularly to improvements in rupture disc arrangements for protection against both excessive pressure and excessive vacuum and specifically for providing rapid equilibration of the pressure within a vessel, with the atmospheric pressure, when pressure inside that vessel has reached predetermined levels of positive or vacuum pressure, causing the rupture disc to break.
In recent years, rupture discs have found increasing use as a highly reliable pressure relief device designed primarily to guard against explosive conditions created when a vessel, for any reason, is subject to excessive internal pressure. In certain applications, the same vessel, for which overpressure protection is indicated, also requires protection against excessive negative guage pressures or a vacuum therein. For example, in the food processing and pharmaceutical manufacturing industries, relatively thin-walled stainless steel tanks are commonly utilized to contain process reactions having a controlled positive internal pressure. These processes are subject to drops in pressure, to a level substantially below atmospheric pressure, and, in the absence of vacuum release, such vessels may buckle inwardly undergoing considerable damage and down time.
For a typical system involving the use of rupture discs, the positive pressure limit will be great compared to the negative pressure limit, for example, 5 to 50 pounds per square inch verses -0.5 pounds per square inch guage. However, accurate control of both is important in order to ensure the safety of personnel and equipment and to ensure process preservation, since the opening of a vessel to air can produce product contamination. Therefore, such manufacturing arrangements have often used devices for protection against both overpressure and underpressure, including complex and expensive mechanical vent valves and vacuum breakers.
Rupture disc assemblies providing some protection against both overpressure and excessive vacuum have been developed. Two such assemblies are described by U.S. Pat. No. 4,079,854 and U.S. Pat. No. 4,119,236, both of which patents are owned by applicant's assignee and the disclosures contained therein are incorporated herein by reference. U.S. Pat. No. 4,079,854 is typical of such devices and comprises a domeshaped relief disc, a flexible seal nested therein, a support structure retaining the flexible sealing disc in an orientation projecting toward the relief disc, and a knife blade seated on the opposite side of the support structure from the seal. The cutting member, having an elongated sharpened blade edge, is positioned in axially spaced relation from the seal and extends a substantial transverse distance thereacross, with a stay arrangement preventing contact between the knife blade and the flexible seal until rupture of the seal is desired. The stay arrangement is designed to have a resistance to bending or plastic deformation sufficient to maintain the seal out of contact with a blade edge only up to a predetermined, and relatively small, differential pressure applied in the appropriate direction. However, when the appropriate pressure is applied, the sealing disc forces the support structure to invert, and is cut by the knife blades, thus opening up a channel to permit fluid flow through the rupture disc assembly.
In practice, rupture discs of the above described type react very well when a large positive pressure is placed upon them in the direction of the concave surface of the relief disc, and they also rapidly invert and rupture when opposite, relatively small, pressures are applied in the direction of the convex face of the relief disc. However, with these devices, once the rupture mode involving the use of the knife blades has taken place, equilibration of the pressure between the inside and the outside of the vessel has been found to be relatively slow. This is believed to be due to the previously unresolved but inherent problem of the cut sealing disc continuing to substantially block passage of fluid through the rupture disc assembly by being supported partially across the opening by the inverted support structure. That is, the inversion pressure differential is insufficient to "snap" the petals past the support, thus, they often "hang up" with the result that they partially block the passage and reduce the equilibration between the ambient or vent pressure and the vacuum in the vessel being protected by the assembly. The major problem with this is that less rapid equilibration can take place and damage to the reaction vessel is more likely because the length of time over which the unsafe condition exists is extended.
Another problem with conventional rupture disc assemblies is that their stay structures have included multiple unassociated appendages which have not necessarily all inverted at the same time. This has resulted in instances of pre-pressure-limit cutting of the seal member and therefore premature opening of the reaction system.
Still another problem with those conventional "two-way" rupture disc assemblies has been that the knife blade assemblies have required expensive and time consuming center welding in order to provide for support of the tip of the knife blade over the center of the opening in the rupture disc. This configuration was considered necessary in order to ensure maximum contact with the sealing member and therefore the greatest amount of cutting.
One limit on possible knife blade configurations has been that cutting patterns which create loose fragments should be avoided, so that portions of the sealing disc do not fall into the reacton vessel.